The DAB/DAB+ SFN On-Channel Repeater (OCR)
A DAB OCR picks a DAB/DAB+ signal out of the air and rebroadcasts it like an SFN-network transmitter. We may have to emphasize that, unlike an FM rebroadcaster, the On-Channel-Repeater system rebroadcasts on the same channel (same DAB frequency block).
However, there will be a back draw. Unwanted feedback will be present between transmit and receive antenna. It will make the system unstable. Three critical actions will be necessary to reduce feedback.
- The (air) coupling between receiving and transmit antenna needs to be as high as possible. In most cases, we will have to use two directional antennas in opposite directions.
- The transmit power should be limited to max. 200W to become a stable system.
- Echo cancellation must reduce the reception of the own signal at the receiver end.
Comparison: Consider the receive antenna as a microphone and the transmit antenna as a loudspeaker. One knows that when the microphone captures too much of the loudspeaker sound, the whole will become unstable. Unstable means the system will start oscillating, and a loud whistle tone will be heard (Larsen effect).
Reducing the feedback of the transmit antenna to the receiving antenna within limits prevents unwanted feedback. Receiving and transmitting antennas should be directional and in opposite directions, needed to keep coupling between antennas to an absolute minimum.
It means that the OCR site must be chosen carefully and should be right between the source transmitter and the region that needs better DAB coverage. Then transmit and receive antennas will be opposite with maximum separation.
How it works
The receiving antenna captures the DAB/DAB+ signal and retransmits it via the transmitting antenna within a very short time-lapse. The retransmission happens on the same DAB/DAB+ channel. It works on the same principle as a DAB SFN network.
The receiving antenna captures the DAB signal and retransmits it via the transmitting antenna within a very small time lapse. The retransmission is done on the same DAB channel. In fact it works on the same principle of a DAB/DAB+ SFN network.
One of DAB/DAB+ specifications was that it should contain the possibility to set up SFN networks (multiple DAB transmitters on the same frequency or DAB channel). The trick they used was the Guard Interval. Assume numerous transmitters on the same frequency, all transmitting perfectly synchronous. However, the receiver will always be at different distances from the individual transmitters and never receive the signals simultaneously.
So, one had to provide a small Interval Guard to solve the time difference at arrival. The Interval Guard has a duration of 246 µs, and all signals arriving within this interval can be detected without problem by the receiver. The speed of light can cover a distance of about 70 km during 0,246 µs. DAB/DAB+ transmitters in an SFN network can be at specific distances from each other before the signals at the receiver exceed the range of the Interval Guard.
Although this also has a limit. So, be careful with SFN transmitters; too far from each other, it won’t work!
So, retransmission on the same channel is possible thanks to the Guard Interval. The guard has been introduced into the system to respect the synchronization rules so that receive and transmit signals of the OCR will reinforce each other, as happens between two independent DAB+ transmitters operating in SFN mode. In other words, let’s call it a remarkable piece of engineering.
See also the European EU FP6 project PLUTO. One of the objectives of the PLUTO project is to make “broadband technologies” such as DVB-T, DVB-H, and DAB more spectrum efficient. The use of small on-channel repeaters is one of the proposed solutions in this project. Most newly developed echo cancellation technologies usually stem from this European project.
There are two necessary conditions to fulfill concerning on-channel DAB repeaters:
- The risk of feedback between transmitting and receiving antennas should be kept as low as possible so that the system does not oscillate (become unstable). In addition to correctly positioning the antennas (directional antennas) and isolating the antennas (far from each other in such a way that they influence each other as little as possible), DAB/DAB+ “on-channel repeaters” use the technology of “echo- cancellation.” In simple terms, one is going to eliminate the own generated transmission signal in the reception signal of the receiver. An adaptive FIR (Fast Impulse Response) filter cares for the echo cancellation. We think there is plenty of literature about this on the internet. We want to point out that correlation is a statistical technique used to compare two sets of numbers. Autocorrelation looks for similarities in one set of numbers over specific ∆Tn (echoes) time intervals.
- SFN rules have to be respected, and a few tenths of microseconds can only delay the transmit signal. In other words, as with two independent DAB/DAB+ transmitters, the transmitted signal from two stations must amplify and not eliminate each other!
Therefore, the mathematical process of “autocorrelation” and “adaptive FIR filter” must be carried out quickly! Even swift DSPs (Digital Signal Processors) cannot achieve this in a few tens of µs. Consequently, one has to do the calculations with much faster hardware. It is where the “FPGA” (Field Programmable Gate Array) comes to the rescue. It is a chip with thousands of integrated digital gate circuits. These ports can be connected arbitrarily by programming the chip. This way, we can realize fast hardware circuits. Once the electronic circuit is up and running and the market shows interest. It is possible to make a specific chip for this function for larger print runs. Such chips are called “ASIC” (Application Specific Integrated Circuit). It is the future of electronics and usually also finds its place in DAB/DAB+ technology.